A. A. Saghiri

Storage ring measurement of the C IV recombination rate coefficient

The low-energy C IV dielectronic recombination (DR) rate coefficient associated with 2s → 2p Δn = 0 excitations of this lithium-like ion has been measured with high-energy resolution at the heavy-ion storage ring TSR of the Max-Planck-Institut fur Kernphysik in Heidelberg, Germany. The experimental procedure and especially the experimental detection probabilities for the high Rydberg states produced by the recombination of this ion are discussed in detail. From the experimental data a Maxwellian plasma rate coefficient is derived with ±15% systematic uncertainty and parameterized for ready use in plasma-modeling codes. Our experimental result especially benchmarks the plasma rate coefficient below 104 K where DR occurs predominantly via C III (1s22p4l) intermediate states and where existing theories differ by orders of magnitude. Furthermore, we find that, to within our systematic uncertainty of 15%, the total dielectronic and radiative C IV recombination can be represented by the incoherent sum of our DR rate coefficient and the radiative recombination rate coefficient of Pequignot and coworkers.

Dielectronic recombination in photoionized gas. II. Laboratory measurements for Fe xviii and Fe xix

In photoionized gases with cosmic abundances, dielectronic recombination (DR) proceeds primarily via nlj ) nl@j@ core excitations (*n \ 0 DR). We have measured the resonance strengths and energies for Fe XVIII to Fe XVII and Fe XIX to Fe XVIII *n \ 0 DR. Using our measurements, we have calculated the Fe XVIII and Fe XIX *n \ 0 DR rate coefficients. Signi—cant discrepancies exist between our inferred rates and those of published calculations. These calculations overestimate the DR rates by factors of D 2o r underestimate it by factors of D2 to orders of magnitude, but none are in good agreement with our results. Almost all published DR rates for modeling cosmic plasmas are computed using the same theo- retical techniques as the above-mentioned calculations. Hence, our measurements call into question all theoretical *n \ 0 DR rates used for ionization balance calculations of cosmic plasmas. At temperatures where the Fe XVIII and Fe XIX fractional abundances are predicted to peak in photoionized gases of cosmic abundances, the theoretical rates underestimate the Fe XVIII DR rate by a factor of D2 and over- estimate the Fe XIX DR rate by a factor of D1.6. We have carried out new multicon—guration Dirac- Fock and multicon—guration Breit-Pauli calculations which agree with our measured resonance strengths and rate coefficients to within typically better than We provide a —t to our inferred rate coeffi- (30%. cients for use in plasma modeling. Using our DR measurements, we infer a factor of D2 error in the Fe XX through Fe XXIV *n \ 0 DR rates. We investigate the eUects of this estimated error for the well- known thermal instability of photoionized gas. We —nd that errors in these rates cannot remove the instability, but they do dramatically aUect the range in parameter space over which it forms. Subject headings: atomic dataatomic processesgalaxies: activeinstabilitiesX-rays: general

Dielectronic recombination (via N=2 -> N '=2 core excitations) and radiative recombination of Fe XX: Laboratory measurements and theoretical calculations

We have measured the resonance strengths and energies for dielectronic recombination (DR) of Fe xx forming Fe xix via N ¼ 2 ! N 0 ¼ 2( DN ¼ 0) core excitations. We have also calculated the DR resonance strengths and energies using the AUTOSTRUCTURE, Hebrew University Lawrence Livermore Atomic Code (HULLAC), Multiconfiguration Dirac-Fock (MCDF), and R-matrix methods, four different state-ofthe-art theoretical techniques. On average the theoretical resonance strengths agree to within .10% with experiment. The AUTOSTRUCTURE, MCDF, and R-matrix results are in better agreement with experiment than are the HULLAC results. However, in all cases the 1 � standard deviation for the ratios of the theoretical-to-experimental resonance strengths is &30%, which is significantly larger than the estimated relative experimental uncertainty of .10%. This suggests that similar errors exist in the calculated level populations and line emission spectrum of the recombined ion. We confirm that theoretical methods based on inverse-photoionization calculations (e.g., undamped R-matrix methods) will severely overestimate the strength of the DR process unless they include the effects of radiation damping. We also find that the coupling between the DR and radiative recombination (RR) channels is small. Below 2 eV the theoretical resonance energies can be up to � 30% larger than experiment. This is larger than the estimated uncertainty in the experimental energy scale (.0.5% below � 25 eV and .0.2% for higher energies) and is attributed to uncertainties in the calculations. These discrepancies makes DR of Fe xx an excellent case for testing atomic structure calculations of ions with partially filled shells. Above 2 eV, agreement between the theoretical and measured energies improves dramatically with the AUTOSTRUCTURE and MCDF results falling within 2% of experiment, the R-matrix results within 3%, and HULLAC within 5%. Agreement for all four calculations improves as the resonance energy increases. We have used our experimental and theoretical results to produce Maxwellian-averaged rate coefficients for DN ¼ 0D R of Fexx. For kBTe & 1 eV, which includes the predicted formation temperatures for Fe xx in an optically thin, low-density photoionized plasma with cosmic abundances, the experimental and theoretical results agree to better than � 15%. This is within the total estimated experimental uncertainty limits of .20%. Agreement below � 1 eV is difficult to quantify due to current theoretical and experimental limitations. Agreement with previously published LS-coupling rate coefficients is poor, particularly for kBTe . 80 eV. This is attributed to errors in the resonance energies of these calculations as well as the omission of DR via 2p1=2 ! 2p3=2 core excitations. We have also used our R-matrix results, topped off using AUTOSTRUCTURE for RR into J � 25 levels, to calculate the rate coefficient for RR of Fe xx. Our RR results are in good agreement with previously published calculations. We find that for temperatures as low as kBTe � 10 � 3 eV, DR still dominates over RR for this system. Subject headings: atomic data — atomic processes — methods: laboratory On-line material: machine-readable tables

Recombination in electron coolers

Abstract An introduction to electron–ion recombination processes is given and recent measurements are described as examples, focusing on low collision energies. Discussed in particular are fine-structure-mediated dielectronic recombination of fluorine-like ions, the moderate recombination enhancement by factors of typically 1.5–4 found for most ion species at relative electron–ion energies below about 10 meV, and the much larger enhancement occurring for specific highly charged ions of complex electronic structure, apparently caused by low-energy dielectronic recombination resonances. Recent experiments revealing dielectronic resonances with very large natural width are also described.

Photorecombination of ions: search for interference effects, recombination at low energies and rate coefficient in plasmas

In the search for interference between radiative and dielectronic recombination (RR and DR), absolute recombination rate coefficients for Ar-like ions have been measured at the Heidelberg Test Storage Ring in the centre-of-mass energy range 0-80 eV. The dominant recombination channels beside RR is DR via the formation of (33d ) and (33d ) intermediate doubly excited states. The DR rate coefficient in plasmas is inferred from this measurement. It is about a factor of 3 lower than the previously available result based on a semi-empirical calculation. Asymmetric lineshapes due to quantum mechanical interference as recently predicted theoretically for isoelectronic ions have not been observed. The broad (3) DR resonance expected on the basis of multi-configuration Hartree-Fock calculations at 3.0 eV with a width of 1.3 eV appears to be shifted towards zero centre-of-mass energy where an unexplained recombination rate enhancement of a factor of 2 beyond the sum of RR and DR rates is observed.

Dielectronic recombination of ground-state and metastable Li+ ions

Dielectronic recombination has been investigated for -n=1 resonances of ground-state Li+(1s2) and for -n=0 resonances of metastable Li+(1s2s 3S). The ground-state spectrum shows three prominent transitions between 53 and 64 eV, while the metastable spectrum exhibits many transitions with energies <3.2 eV. Reasonably good agreement of R-matrix, LS coupling calculations with the measured recombination rate coefficient is obtained. The time dependence of the recombination rate yields a radiative lifetime of 52.2±5.0 s for the 23S level of Li+.

Dielectronic recombination of Li-like fluorine ions

Some recent new developments in dielectronic recombination (DR) measurements at heavy-ion storage rings are presented. A DR spectrum measured at CRYRING in Stockholm of F6+ is used to illustrate a few points regarding this kind of measurements. From the spectrum a transversal temperature of the adiabatically expanded electron beam of 3.0(5) meV is deduced, which is slightly higher than expected.

Recent dielectronic recombination experiments

New recombination experiments with merged cold beams of electrons and atomic ions have been carried out at the storage ring facilities TSR in Heidelberg, ESR in Darmstadt, and CRYRING in Stockholm. A brief overview is given on the recent activities in which the Giessen group was engaged. Topics of this research were dielectronic recombination (DR) of astrophysically relevant ions, recombination of highly charged ions with respect to cooling losses in storage rings, field effects on DR, search for interference effects in photorecombination of ions, correlation effects in DR of low-Z ions, spectroscopy of high-Z ions by DR, and lifetimes of metastable states deduced from DR experiments.

Measurements of low temperature dielectronic recombination in L-shell iron for modeling X-ray photoionized cosmic plasmas

The iron L-shell ions (Fe17+ to Fe23+) play an important role in determin- ing the thermal and ionization structures and line emission from photoionized plasmas. Current uncertainties in the theoretical low temperature dielectronic recombination (DR) rate coefficients for these ions significantly affect our ability to model and inter- pret the line emission from observations of photoionized plasmas. To help resolve this issue, we have initiated a program of laboratory measurements to produce reliable low temperature DR rates for the L-shell iron. Here we present some of our recent results and discuss some of the astrophysical implications.

Dielectronic Recombination of Fe XXIII Forming Fe XXII: Laboratory Measurements and Theoretical Calculations: Data

We havemeasured resonance strengths and energies for dielectronic recombination (DR) of Mg-like Fe xv forming Al-like Fe xiv viaN 1⁄4 3 ! N 0 1⁄4 3 core excitations in the electron-ion collision energy range 0Y45 eV. All measurements were carried out using the heavy-ion test storage ring at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We have also carried out new multiconfiguration Breit-Pauli (MCBP) calculations using the AUTOSTRUCTURE code. For electron-ion collision energies P25 eV we find poor agreement between our experimental and theoretical resonance energies and strengths. From 25 to 42 eV we find good agreement between the two for resonance energies. But in this energy range the theoretical resonance strengths are 31% larger than the experimental results. This is larger than our estimated total experimental uncertainty in this energy range of 26% (at a 90% confidence level). Above 42 eV the difference in the shape between the calculated and measured 3s3p( P1)nl DR series limit we attribute partly to the nl dependence of the detection probabilities of high Rydberg states in the experiment. We have used our measurements, supplemented by our AUTOSTRUCTURE calculations, to produce a Maxwellian-averaged 3 ! 3 DR rate coefficient for Fe xv forming Fe xiv. The resulting rate coefficient is estimated to be accurate to better than 29% (at a 90% confidence level) for kBTe 1 eV. At temperatures of kBTe 2:5Y15 eV, where Fe xv is predicted to form in photoionized plasmas, significant discrepancies are found between our experimentally derived rate coefficient and previously published theoretical results. Our newMCBP plasma rate coefficient is 19%Y28% smaller than our experimental results over this temperature range. Subject headinggs: atomic data — atomic processes — galaxies: active — galaxies: nuclei — plasmas — X-rays: galaxies